The present invention relates to an infrared spectrometer with a housing comprising an optical light source and optical interferometer to divide the input light beam emitted by a light source into two partial beams and to generate a variable optical retardation between the two partial beams as well as to recombine these beams to one output light beam. Furthermore, the infrared spectrometer includes a sample position to accept a sample at which the sample is either irradiated or transmitted by the output light beam, and an optical single-component detector to analyze the detector light beam signal emitted by the sample, a dedicated component to digitize the detector signal available at the detector output, and means to process the digitized detector signal.
Corresponding infrared spectrometers (FTIR spectrometers) are manufactured and distributed by the applicant, e.g. the “IFS 66/S” spectrometer described in applicant's IFS 66/S brochure, dated December 1999.
The prior-art FTIR spectrometers have been used to measure infrared spectra either in transmission or reflection. They have both a compact and a modular design, i.e. the components can be easily replaced, if required, the user can easily switch between different components, e.g. between several sources, detectors or filters. A further outstanding advantage of these spectrometers is that they can be used in connection with external sources or detectors by means of inputs and outputs. They can measure spectra of sunlight, or by means of fiber optics or conventional optics they can be connected to an infrared microscope. The IR light generated within the spectrometer first passes the interferometer, is then directed to an outlet and finally reaches an IR microscope where it illuminates a sample. The light transmitted from or reflected by this sample then reaches an external detector. In case of the microscope, this detector can be an imaging detector array, e.g. FPA array. Its output signals will be digitized, cached and processed by a computer to generate a two-dimensional spatially resolved spectrum. Frequently, several permanently built-in and/or external detectors are used which can be replaced or between which one may switch e.g. by means of a hinged mirror.
According to chapter 4 “New Designs” of the lecture No. 2001 (A. Adam and M. Goodnough) at the Pittsburgh Convention 2000 it is known that analogue-to-digital converters (ADCs) can directly be mounted on an FPA detector chip, thus reducing operating costs and simplifying the system complexity. In the future, this may result in extremely high frame rates of >30 kHz for a 128×128 array. Therefore, this type of detectors is suitable for precise imaging remote-sensing systems incorporating pulsed lasers.
In FTIR spectroscopy an optical signal is frequency-modulated by the interferometer. This frequency-modulated signal is measured by a detector, converted into a corresponding analogue voltage, then digitized and divided into its spectral components by Fourier transformation and displayed.
Instead of single-element detectors, imaging IR spectroscopy usually uses FPA detectors consisting of 64×64 or more elements. These elements undergo a short-time exposure, and subsequently each pixel will be connected by means of an analogue switch to a digitizing unit (ADC) and digitized. This kind of scanning and conversion of a frame with e.g. 64×64 pixels corresponds to one interferogram data point generated by conventional (non-imaging) FTIR spectroscopy. In order to be able to measure data with otherwise identical methods, the electrical bandwidth of the analogue and digital signal needs to be multiplied by the number of pixels.
In contrast to detector arrays which enable digitization of the analogue measuring voltage directly on the detector chip, prior-art single-element detectors transmit the measuring signals emitted by the detector to a remote circuit board which is equipped with an analogue-digital converter (ADC) and, frequently, also with processing digital electronics. Only after the analogue/digital conversion will the further digital processing be performed. The analogue signal path can either be ac connected to fully utilize the dynamic of the analogue/digital converter or dc connected in order to acquire the full information. Ideally, both signals are transferred to the converter to fully benefit from the ADC dynamic and to completely acquire the information. The analogue signal path within the spectrometer may well be in the order of several decimeters.
Infrared spectrometers of the kind mentioned hereinbefore using analogue measuring signal transmission are still unsatisfactory with respect to their susceptibility to external interferences. This invention intends to further reduce this susceptibility.